Method of transfer molding electronic packages and packages produced thereby

ABSTRACT

A printed wiring board with either a pin grid array, a ball grid array, a land grid array, etc. of electrical contacts is prepared with a heat sink in the usual manner. A passage is provided either in the printed wiring board or in the heat sink so that during the transfer molding process, fluid molding compound passes latitudinally under the heat sink into a cavity below the heat sink to encapsulate the package.

This application is a divisional of application Ser. No. 08/452,024,filed May 26, 1995, now U.S. Pat. No. 5,652,463.

FIELD OF THE INVENTION

This invention relates in general to improved techniques forencapsulating objects, such as electronic devices and components and,more particularly, to improved means and methods for transfer molding ofsemiconductor or other electronic devices.

BACKGROUND OF THE INVENTION

In the fabrication of semiconductor and electronic devices there is anongoing need to reduce packaging costs. Package sizing is alsoimportant, especially the profile or height of the package, when mountedto a printed wiring board or printed circuit board. Complicating thesituation is the increasing complexity of electronic components such asintegrated circuits which require a high pin count package toelectrically connect the device to a user system.

Electronic circuits for complex systems such as digital computerstypically are comprised of a multiplicity of interconnected integratedcircuit chips. The integrated circuit chips are made from asemiconductor material such as silicon or gallium arsenide, andmicroscopic circuits are formed on the top surface of the chips byphotolithographic techniques. In a conventional form of construction,the integrated circuit chips are mounted in respective ceramic packages,and the ceramic packages are mounted on a printed wiring board orprinted circuit board.

Plastic integrated circuit packages have evolved as a cost effectivereplacement for ceramic packages. Modern laminate based molded plasticpackages offer electrical, thermal and design performance that matchesand often times exceeds that of ceramic packages at a lower cost.Electrically, laminate substrates have a clear advantage over co-fireceramic with both lower resistance wiring and lower dielectric constant.Essentially, electrical designs can be implemented in less than half thevolume (and half the number of layers) as an equivalent ceramic baseddesign.

Plastic encapsulation of semiconductor and electronic devices bytransfer molding is a well-known and much used technique. In a typicalsituation, a large number of components or devices are placed in an openmulti-cavity mold, one or more devices in each cavity. When the mold isclosed the two mold portions, usually called "platens" or "halves", cometogether around the devices. The many cavities in the mold are connectedby a tree-like array of channels (i.e., runners) to a central reservoir(i.e., pot) from which the plastic is fed. Usually, "gates" (i.e.,constricted channels) are placed just at the entrance to each cavity tocontrol the flow and injection velocity of the plastic into the cavity,and to permit easier removal from the finished part of the materialwhich has solidified in the runners.

Typically, powdered or pelletized plastic is placed in the centralreservoir and compressed by a ram. The mold and reservoir are usuallyhot. The combination of heat and pressure causes the plastic to liquifyand flow through the runner-tree and gates into the individual moldcavities, where it subsequently hardens. The mold halves are thenseparated and the encapsulated parts are removed and trimmed of excessplastic left in the runners and the gates.

Heat sinks have been added to plastic molded electronic packages asdescribed in U.S. Pat. No. 4,868,349 issued to Chia. The heat sinkprovides more efficient heat removal in a plastic molded electronicpackage. The heat sink in Chia is attached in the following manner. Aprinted wiring board is prepared to have a plurality of package contacts(i.e., pins) secured to extend from one face in the form of a pin gridarray. The printed wiring board has a cut-out region in its center and acopper heat sink is secured to the board to span the cut-out region onthe opposite face of the board. A semiconductor (or electronic) deviceis then secured to the heat sink in the cavity created by the cut-outregion. The bonding pads of the semiconductor device are thenelectrically connected to the traces on the printed wiring on the boardthat are joined to the package contacts. The printed wiring board ofChia is also provided with a plurality of holes located next to andoutboard of the heat sink.

The assembly is then located in a transfer mold composed of a pair ofopposing platens. A first platen has a first cavity that accommodatesthe package contacts and this cavity includes edges that bear againstthe printed wiring board adjacent to the pins. A second cavity islocated in the first platen to span the holes in the board adjacent tothe heat sink. The second or opposing platen has a cavity that spans theprinted wiring board and is deep enough to accommodate the heat sink. Italso contains a series of ribs that engage the printed wiring board soas to precisely locate it inside the platen and to press against theboard periphery so as to force it against the first platen. It alsocontains runners and gates through which fluid plastic can be entered ina transfer molding process.

When fluid plastic is forced through the runners and gates, it entersthe second cavity and covers the board adjacent to the heat sink but isprecluded from coating the heat sink by virtue of contact with theplaten cavity face. The fluid plastic fills the platen cavity around theheat sink and also flows through the holes in the board thereby to coverthe semiconductor device inside the second cavity. The mold becomesfilled with plastic which acts to encapsulate the printed wiring boardand the housed semiconductor device. After transfer molding and partialcure of the encapsulant, the device can be removed from the mold and thecure completed. After curing, any flash is removed in the usual mannerand the device is ready for use.

The Chia method has some particular advantages but likewise has certainlimitations. First, the thickness of the printed wiring board can varyby as much as ±5 mils, whereas the height of the cavity between thefirst platen and second platen when they are brought together is fixed.The results being that if the printed wiring board is 5 mils too thick,as the mold is closed the heat sink and printed wiring board will bedeflected into the second cavity, particularly because the heat sinkwidth is smaller than the second cavity width, therefore there is nosupport below the printed wiring board in the heat sink area to resistthe deflection. Then after the mold is opened, the pressure is releasedand the printed wiring board and heat sink return to their initialposition. As a result, the heat sink is no longer flush with the top ofthe plastic encapsulant and cracks develop in the plastic at the heatsink-plastic interface. If the printed wiring board is 5 mils too thin,as the mold is closed there will be a 5 mil gap between the top of theheat sink and the inside surface of the cavity above the heat sink. As aresult, plastic will flow over the heat sink and encapsulate it.

A second limitation of the Chia package is that the heat sink has to berelatively small so that the access holes through the board will beinboard of the package pin contacts but not covered up by the heat sink.The relatively small heat sink limits cooling of the package and limitsthe size of an external heat sink that a customer may wish to add to thepackage for additional cooling.

For integrated circuit (IC) plastic packaging, the IC chip is usuallyencapsulated because it is the lowest cost. Using a lid forencapsulating is typically used if the IC chip cannot tolerate thestresses of mold or liquid compound on the surface of the die.Encapsulation using liquid compound is another common method. Thisprocess is also known as "glob top" or potting.

The difference between glob top and potting methods is that the glob topis "self-crowning" liquid encapsulation. The potting method is morecosmetically appealing than the glob top method, but the potting methodneeds a "dam" or potting ring to prevent the encapsulating liquid fromflowing in unwanted areas and therefore adds cost. The potting ring istypically separate from the printed wiring board and attached using anadhesive. Because the potting ring is separate from the printed wiringboard, there is added cost of handling two individual pieces and theadhesive to make it one unit. An alternative method of applying apotting ring is screen printing. However, there are height and heighttolerance limitations.

SUMMARY OF THE INVENTION

It is an object of the present invention to overcome the limitations ofthe prior art devices and methods. One object of the present inventionis to provide a method and a package produced from that method that hassuperior heat transfer characteristics. It is another object of thepresent invention to provide a method that produces high yield for themanufacture of transfer molded electronic packages having heat sinksattached. Another object of the present invention is to provide acost-effective method of producing a potting ring on a transfer moldedpackage.

These and other objectives are achieved in the following manner. Aprinted wiring board with either a pin grid array, a ball grid array, aland grid array, etc. of electrical contacts is prepared with a heatsink in the usual manner. A passage is provided either in the printedwiring board or in the heat sink so that during the transfer moldingprocess, fluid molding compound passes latitudinally under the heat sinkinto a cavity below the heat sink.

In accordance with one embodiment of the present invention, there isprovided a molded electronic package, comprising a printed wiring boardhaving a first surface, a second surface and plated vias, said printedwiring board having a window therethrough and a heat sink attached tothe first surface to cover the window, said electronic package havingpassage means for allowing fluid molding compound to move latitudinallyunder the heat sink from the first surface of the printed wiring board,and an encapsulant covering the first surface of the printed wiringboard around the heat sink, said encapsulant extending through thepassage means and filling a cavity below the first surface.

In accordance with another embodiment of the present invention, there isprovided a molded electronic package, comprising a printed wiring boardhaving a first surface, a second surface and plated vias extendingthrough the board, said printed wiring board having a windowtherethrough and a heat sink attached to the first surface to cover thewindow, an electronic device attached to the heat sink within thewindow, and means for connecting the electronic device electrically tothe plated vias, said electronic package having passage means forallowing fluid molding compound to move latitudinally under the heatsink from the first surface of the printed wiring board, and anencapsulant covering the first surface of the printed wiring boardaround the heat sink, extending through the passage means andencapsulating the electronic device and the connections thereto.

In one of the method aspects of the present invention, there is provideda method for making a molded electronic package comprising the steps offorming a printed wiring board having plated via holes and a windowtherethrough, providing a heat sink for attaching to the printed wiringboard, attaching the heat sink to a first surface of the printed wiringboard to cover the window, attaching an electronic device in the windowto the heat sink, connecting the electronic device electrically to theplated via holes, and encapsulating the electronic package bytransferring fluid molding compound latitudinally through a passageunder the heat sink into the window to encapsulate the electronic deviceand the connections thereto.

In another one of the method aspects of the present invention, there isprovided a method for making a molded package having a potting ringcomprising the steps of forming a printed wiring board having plated viaholes and a window therethrough, providing a heat sink for attaching tothe printed wiring board, attaching the heat sink to a first surface ofthe printed wiring board to cover the window, and encapsulating thepackage by transferring fluid molding compound latitudinally through apassage to form a potting ring on the second surface of the board.

In yet another one of the method aspects of the present invention, thereis provided a method for making a molded electronic package comprisingthe steps of forming a printed wiring board having plated via holes anda window therethrough, providing a heat sink for attaching to theprinted wiring board, the heat sink having a groove in a first surfaceextending from an edge of the heat sink toward its center, attaching theheat sink to the first surface of the printed wiring board to cover thewindow and such that the groove extends into the window, attaching anelectronic device in the window to the heat sink, connecting theelectronic device electrically to the plated via holes, andencapsulating the electronic package by transferring fluid moldingcompound over the first surface of the board and latitudinally throughthe groove in the heat sink into the window to encapsulate theelectronic device and the connections thereto.

BRIEF DESCRIPTION OF THE DRAWING

Many objects and advantages of the present invention will be apparent tothose of ordinary skill in the art when this specification is read inconjunction with the attached drawings wherein like reference numeralsare applied to like elements and wherein:

FIG. 1 is a cross-sectional view of a substrate in a transfer mold inaccordance with one embodiment of the present invention;

FIG. 2 is top plan view of the substrate of FIG. 1;

FIG. 3 is an encapsulated electronic package in accordance with oneembodiment of the present invention;

FIG. 4 is a bottom plan view of the electronic package of FIG. 3;

FIG. 5 is a cross-sectional view of a substrate in a transfer mold inaccordance with another embodiment of the present invention;

FIG. 6 is a cross-sectional view of a substrate in a transfer mold inaccordance with yet another embodiment of the present invention;

FIG. 7 is a bottom plan view of the substrate of FIG. 6;

FIG. 8 is a top plan view of the substrate of FIG. 6;

FIG. 9 is an encapsulated package with a molded potting ring inaccordance with one embodiment of the present invention;

FIG. 10 is a cross-sectional view of a substrate in a transfer mold inaccordance with another embodiment of the present invention;

FIG. 11 is a cross-sectional view of a substrate in a transfer mold inaccordance with yet another embodiment of the present invention;

FIG. 12 is a cross-sectional view of a substrate with passage means inaccordance with another embodiment of the present invention; and

FIG. 13 is a cross-sectional view of yet another embodiment of thepassage means.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

With reference to FIG. 1, the starting element is a laminated substrate10. Substrate 10 is a conventional printed wiring board (PWB), typicallyof the multi-layer variety. It includes a series of plated vias (i.e.,through holes) 11 into which electrical contacts (e.g., solder balls,pads, pins, etc.) are secured usually by soldering to form a ball gridarray, pin grid array, land grid array, etc. A window 13 is cut throughthe center of the PWB. In the particular embodiment of FIG. 1, window 13passes through layers 5, 6 and 7 with the portion in layers 6 and 7being larger than the portion in layer 5.

Passage or feed means 14 is provided in substrate 10. In FIG. 1, passagemeans 14 is a slot or channel 15 cut in the top surface 16 of substrate10 (see FIG. 2 in which the via holes are not shown for simplicity).However, it is important to note that the passage means can be, but notlimited to, a hole, a groove, orifice, duct, notch or any number ofconfigurations that allow fluid molding compound (e.g., plastic) to passlatitudinally under heat sink 19. Latitudinal for the present inventionincludes movement that is generally side to side, including length-wiseor width-wise movement as opposed to height-wise or thickness-wisemovement. However, latitudinal also includes angular side to sidemovement that can be represented by a passage that extends transverselythrough the substrate at the same time as descending through thethickness (i.e., the height) of the substrate. Likewise, angular side toside movement can be represented by a passage that extends transverselythrough the substrate and at the same time have an increasing depth(i.e., channel depth). As will be discussed below, the passage means canalso extend latitudinally for a distance and then connect with avertical passage through the substrate.

As will be described later in more detail, passage means 14 plays animportant role in the molding process, in particular it allows for theimprovements of the present invention over the prior art. Heat sink 19is secured to top surface 16 of substrate 10 over window 13. Heat sink19 is typically a slab of cooper covered with a thin anticorrosionnickel or gold plate and is secured to substrate 10 by an adhesive.Substrate 10 includes conductive traces that join vias 11 to an array ofbond fingers on bottom surface 17 of the board that surround theperiphery of hole 13. Once heat sink 19 is secured to substrate 10 thepackage forms a cavity available from the bottom side or surface 17 formounting an electronic device 20 therein.

Electronic device 20 is typically a chip having an integrated circuit onthe top thereof, but can be other active devices such as diodes,transistors, etc. Likewise, multiple chips or other devices can beattached to one laminated substrate. In one embodiment, the integratedcircuit of electronic device 20 is attached to the laminated substrateby bonding wires 21 from bonding pads on the top of the chip to bondfingers 18 on one tier of the laminated substrate and by bonding wires21A to bond fingers 18A on a second tier of the laminated substrate. Thebonding wires are typically 25 micrometer diameter gold wires, howeveras one of ordinary skill in the art is aware various diameters andmaterials, such as aluminum or other metals, can be used especially forhigh-current-power devices.

In one embodiment, the wires are bonded to the bond fingers using athermosonic bonding process which uses a combination of heat(approximately 150° C. to 200° C.) and ultrasonics (approximately 60 to70 kHz) to obtain a good mechanical bond and a very low resistanceelectrical contact. An ultrasonic bonding process which uses justultrasonics (approximately 60 to 70 kHz) can also be used. In thethermosonic and similar types of bonding processes, the end of the wirebecomes expanded to about two to three times its original diameter; sothe large bond fingers are advantageous. After the electronic device hasbeen attached, the laminated substrate can be used in a conventionalplastic package to encapsulate the electronic device.

The following is an example of the practice of the present invention.Those of skill in the art will recognize that other embodiments of thepresent invention are also suitable. The mold used for the presentinvention is disclosed in U.S. patent application Ser. No. 08/452,130filed concurrently herewith as Attorney Docket No. 011608-013 andentitled "APPARATUS FOR ENCAPSULATING ELECTRONIC PACKAGES" which iscommonly owned and incorporated herein by reference in its entirety.Lower platen 23 is placed in a mold press and substrate 10 is placedover lower platen 23. Upper platen 22 is aligned over lower platen 23 sothat upper mold cavity 25 and lower mold cavity 27 are formedsurrounding substrate 10. FIG. 1 shows the substrate located betweenmold platens 22 and 23, which are employed in the transfer moldingoperation. As used herein the words "upper platen" and "lower platen" or"upper mold platen" and "lower mold platen" refer to the two separableportions of the mold used to define the enclosed mold cavities in whichmolding is to occur. The words "upper" and "lower" are used for ease ofdescription and do not imply a required orientation in space, since themolds may be readily designed to operate in either top transfer (ram ontop) or bottom transfer (ram on bottom) configurations without affectingtheir basic function. All of the figures only show a single position inthe transfer mold which would ordinarily involve a plurality of suchpositions so that a relatively large number of devices can besimultaneously molded. Well known mold features such as ejector pinswhich facilitate removal of the finished part(s) are omitted forclarity. Those of skill in the art will recognize that such a featureand/or others may be used in practice. The transfer mold is employed ina conventional transfer molding process. Alignment bumps 43 can beprovided on substrate 10 to center the substrate in the platen.

In the transfer molding operation, a predetermined volume of fluid(heated) molding compound at least sufficient to fill the net volume ofmold cavities 25 and 27 plus the volume of transfer means 14 and viaholes 11 is forced into hole 24 at approximately 500 psi. The moldingcompound is typically a thermoset plastic. The molding compound can beany of a number of materials known to one of ordinary skill in the art,including but not limited to those disclosed in Sporck, U.S. Pat. No.3,838,094, Shiobara et al., U.S. Pat. No. 4,859,722, and Jusky et al.,U.S. Pat. No. 5,132,778, all of which are incorporated herein in theirentirety.

The fluid molding compound passes into upper mold cavity 25 through gate26. In upper mold cavity 25, the arrows show the resulting moldingcompound flow. The molding compound flows around heat sink 19, overupper surface 16 of substrate 10 and around sides 8 of substrate 10 toencapsulate the top and sides of the package. It is important that thevelocity at which the liquid molding compound is injected into the moldcavities be controlled so as not to exceed a maximum injection velocity.The limit on the maximum injection velocity is required in order toavoid forming voids in the molding compound or having the rapidly movingmolding compound damage fragile elements of the electronic device, suchas bonding wires 21,21A or the semiconductor chip. The maximum injectionvelocity may be readily determined by experiment. It is also importantthat the injection molding time be less than the solidification time.This second requirement imposes a lower limit upon the injectionvelocity.

As mentioned above, passage means 14 provides particular advantages.Passage means 14 allows fluid molding compound to move latitudinally orlaterally under heat sink 19 through substrate 10 into lower mold cavity27 in lower mold platen 23 to encapsulate electronic device 20, bondingwires 21, 21A, and bond fingers 18, 18A. The latitudinal movement ofmolding compound under the heat sink means that a large heat sink can beplaced over the top of the package for greater heat transfer. In fact,the heat sink could cover the entire top surface of the substratewhereby passage means 14 would extend all the way to the side of thesubstrate (FIG. 12).

Passage means 14 has a variable size. The passage can be wide andshallow or, alternatively, skinny and deep. The important characteristicis to have a cross-sectional area that is of a proper size to preventspiral flow or blockage in the passage. Preventing these events is wellwithin the ordinary skill of one skilled in the art. For example, atypical size can be 20 mils deep and 60 mils wide resulting in across-sectional area of 1200 square mils. Variations in the size of thepassage can be made depending on, but not limited to, such factors as:dimensions of mold cavity; dimensions of substrate and heat sink;viscosity of molding material; composition of molding material; etc. Itis preferable that there be no circuitry or via holes below the passage.However, a passage can pass through via holes without a problem. Asshown in FIG. 2, passage means 14 runs diagonally from the corner of thesubstrate to window 13. However, the passage means can run from anydirection in the substrate and need not extend all the way to the edgeor to the window as will be discussed in more detail below. There canalso be more than one passage means in accordance with anotherembodiment of the present invention.

The upper platen 22 includes a plug 33 that presses against heat sink 19so as to preclude molding compound from flowing over the upper portionof the heat sink. Plug 33 is pressed against heat sink 19 using biasingmeans 39 located in cavity 41 of upper platen 22. Biasing means 39 alsopresses substrate 10 onto surface 45 of lower platen 23 so that moldingcompound is prevented from flowing across the bottom surface of thesubstrate except in the area of mold cavity 27. A spacer (not shown) canbe placed in cavity 41 between biasing means 39 to adjust the amount ofpressure the biasing means exerts against heat sink 19. Likewise, thebiasing means can be changed out with a biasing means of differentstrength.

The use of a relatively large heat sink 19 (i.e., having a greater widththan the width of the mold cavity) in the molding process of the presentinvention has particular advantages over the prior art. As can be seenin FIG. 1, heat sink width A is greater than mold cavity width B. Inaddition, plug 33 has a greater width than the heat sink. As a result,when upper mold platen 22 and lower mold platen 23 are brought together,substrate 10 is supported by the lower mold platen (illustrated byarrows 49 in FIG. 1) as biased plug 33 exerts force on the substratethrough heat sink 19. In this way, the center portion of the substratewill not deflect (as it does in the prior art) because the pressure ofthe plug is transmitted from the heat sink directly through the board tothe lower platen (instead of being transmitted through the board to anopen mold cavity where there is no structure to support it).

The biased plug adds another particular advantage in that the biasingmeans 39 can compensate for any tolerance variations in the thicknessesof the substrate and/or heat sink. As can be seen in FIG. 1, height Cbetween top surface 45 of lower platen 23 and top surface 47 of upperplaten 22 is a fixed distance when the two platens are brought togetherfor the molding process. However, the thickness of the substrate and/orheat sink can vary by several mils. These variations are compensated forwith biasing means 39. For example, if the substrate is too thick, heatsink 19 deflects biasing means 39 and plug 33 upward so that unduepressure is not exerted on the substrate as the two platens are broughttogether. On the other hand, if substrate 10 is too thin, biasing means39 deflects and forces plug 33 downward onto the upper surface of heatsink 19, thus preventing a gap between the lower surface of the upperplaten and the upper surface of the heat sink wherein molding compoundmight have flowed over the heat sink.

In the transfer molding art, it is a good rule of thumb that the fluidmolding compound will not flow into a gap or recess smaller than about0.025mm. Thus, vents 28 and 28A having a diameter of about 0.01 mm willrelieve air pressure within the mold while precluding the flow ofmolding compound therethrough.

The finished transfer molded pin grid-array is shown in FIG. 3 incross-section. The molded package typically starts to cure in the mold.Then, the package is removed from the mold and cured or cross-linked atabout 175° C. for four hours. The molding compound hardens intoencapsulant 37 that surrounds the top surface and sides of substrate 10,as well as covers the electronic device, bonding wires and bond figures.Electrical contacts (i.e., pins) 12 are secured in vias 11 by solderingusually.

FIG. 4 is a bottom view of the finished transfer molded pin grid arraypackage illustrating that the encapsulant fills the center cavity toencapsulate the electronic device therein and extends over and aroundthe edges of the substrate. In another embodiment of the presentinvention, solder balls are attached to the via holes to form a ballgrid array. Likewise, land grid arrays and other contact configurationscan be formed in accordance with the present invention.

The lower platen 23 also can have recesses (not shown in FIG. 1) locatedat the four corners of lower mold cavity 27. The molding compound canthen form protrusions (not shown) from the bottom surface 35 ofencapsulant 37 so that when the package is mounted on a conventionalprinted circuit board it will seat with the protrusions in contact withthe board while the central portion of encapsulant 37 will clear theboard surface. Because the protrusions are located at the four cornersof the package the central region is accessible to cleaning fluid afterthe package is soldered in final assembly.

Referring back to FIG. 1, it is to be understood that while heat sink 19is shown as a flat slab, it can take other forms. For example, anintegral threaded boss can be included on the top of the heat sink. Sucha boss can facilitate the attachment of heat fins or some other form ofcooling mechanism. When such a boss is present on heat sink 19 the plug33 will contain an accommodating recess for the boss. Whether such aboss and recess is present, the close spacing between heat sink 19 andplug 33 around the periphery of the heat sink will preclude moldingcompound from covering the heat sink face during molding.

In one embodiment of the present invention, lower platen 23' can beused. Lower platen 23' (FIG. 5) has a plurality of recesses or cavities51, 51A for receiving electrical contacts 12. The upper platen in thisembodiment is identical to the upper platen discussed previously and themolding process is the same. The electrical contacts in the recesses canbe used to center the substrate, therefore, there is no need for otherelements (such as alignment bumps 43 in FIG. 2) to be used for alignmentpurposes.

Another embodiment of the present invention is shown in FIG. 6, upperplaten 22 is the same as described above but the lower platen andpassage means are different for the purpose of forming a potting ring onbottom surface 17 of substrate 10' instead of encapsulating anelectronic device. Potting ring (or dam ring) 55 is formed on the bottomof substrate 10' at the same time that encapsulant 37 is formed aroundthe heat sink and sides of the substrate so that encapsulated package 53can be used for the later attachment of an electronic device to bottomsurface 57 of heat sink 19 and encapsulated with a liquid encapsulant(FIG. 9). Dam ring 55 prevents the liquid encapsulant from spreading toundesired areas before it hardens. As a result of molding the pottingring to the substrate, there is no need for handling a potting ringseparate from the substrate. Some of the advantages are: lower cost dueto less material and material handling; no limitations on potting ringheight and tolerance; and better cosmetics of the molded package.

As mentioned above, the lower platen used for this embodiment isslightly different than lower platens 23 and 23' discussed previously.Lower platen 59 in FIG. 6 has a ringed mold cavity 61 formed therein.Typically, mold cavity 61 is square or rectangular shape to match theshape of window 13 in the substrate, however, the mold cavity can beround or any other shape. Likewise, the cross-section of the mold cavityis similar to an inverted, truncated pyramid but the cross-section couldbe any of a large number of shapes. Lower platen 59 and relatively largeheat sink 19 have the same particular advantages as discussed above suchas preventing deflection in the center of the substrate.

Passage means 14 in substrate 10' is a shortened passage or groove 15'(of similar configuration and size as passage 15 discussed previously)and opening 63 extending through substrate 10' from passage 15' tosecond surface 17 of the substrate. FIG. 8 is top view of substrate 10'before encapsulation showing passage 15' extending from the corner ofthe substrate to opening 63. As before, passage 15' can run from anydirection in the substrate and need not extend all the way to the edgeof the substrate. There can also be more than one passage means inaccordance with another embodiment of the present invention. FIG. 7 is abottom view of substrate 10' extending through bottom surface 17 of thesubstrate.

Passage 15' traverses latitudinally under heat sink 19 to opening 63.The heat sink could cover the entire top surface of the substratewhereby passage means 14 would extend all the way to the side of thesubstrate as is shown in FIG. 12. Transfer means 14 shown in FIG. 12 isformed in or routed in layer 6 of the substrate. Potting ring 55 ismolded by incorporating the opening through the substrate inside thepassage area (FIG. 9). FIG. 6 shows that during the molding process, thefluid molding compound passes into upper mold cavity 25 through gate 26.In upper mold cavity 25, the arrows show the resulting molding compoundflow. The molding compound flows around heat sink 19, over upper surface16 of substrate 10 and around sides 8 of substrate 10' to encapsulatethe top and sides of the package. The molding compound flows around theedges and top surface of substrate 10' into passage 15' and down throughopening 63 into mold cavity 61. FIG. 9 shows the completed encapsulatedpackage 53 with molded potting ring 55. Electrical contacts 12 in theform of solder balls to form a solder ball grid array have beenelectrically connected to via holes 11. Electrical contacts 12 could bepins, lands, etc. as known in the art.

FIG. 13 shows an embodiment of the present invention in which openings77 and 79 can be made in the top and bottom of the substrate and areincorporated into transfer means 14 by joining with passage 15". Passage15" is formed in or routed in layer 6 of the substrate, but it should beunderstood that it could be in any layer or multiple of layers.

In yet another embodiment of the present invention, lower platen 65 canbe used for the molding process (FIG. 10). Lower platen 65 includes plug67 that presses against lower surface 17 of substrate 10 such that thetop surface of heat sink 19 is pressed into contact with surface 47 ofupper mold cavity 25 in upper platen 75 so as to preclude moldingcompound from flowing over the top surface of the heat sink. Plug 67 isbiased using biasing means 39 located in cavity 69 of lower platen 65.Biasing means 39 also presses surface 71 of plug 67 against surface 71of substrate 10 so that molding compound is prevented from flowingacross the bottom surface of the substrate except in the area of moldcavity 27. Plug 67 is dimensioned to fit in cavity 69 with a gap of lessthan about 0.01 mm along either side so that molding compound will notflow into cavity 69 during the molding process. The use of therelatively large heat sink 19 (i.e., having a greater width than thewidth of the mold cavity) and plug 67 in the molding process of thepresent embodiment has the same advantages as discussed above, such ascompensating for variations in the thicknesses of the heat sink andsubstrate and preventing deflection in the center of the substrate. Itis important to note that biased plug 67 could have a mold cavity likecavity 61 for forming a potting ring instead of mold cavity 27.Likewise, plug 67 could have recesses for accommodating pins or otherelectrical contacts as discussed with reference to the embodiment ofFIG. 5.

FIG. 11 is yet another embodiment of the present invention. The moldplatens are the same as just discussed. In this embodiment, however,passage means 14 is a slot or channel 73 cut in bottom surface 57 ofheat sink 19'. However, it is important to note that the passage meanscan be, but not limited to, a hole, a groove, orifice, duct, notch orany number of configurations that allow fluid molding compound (e.g.,plastic) to pass latitudinally under heat sink 19'. In the transfermolding operation, a fluid (heated) molding compound mixture is forcedinto hole 24 in upper platen 75. The fluid molding compound passes intoupper mold cavity 25 through gate 26. In upper mold cavity 25, thearrows show the resulting molding compound flow. The molding compoundflows around heat sink 19', over upper surface 16 of substrate 10 andaround sides 8 of substrate 10 to encapsulate the top and sides of thepackage.

Passage means 14 allows fluid molding compound to move latitudinally orlaterally under heat sink 19' through substrate 10 into lower moldcavity 27 in lower mold platen 23 to encapsulate electronic device 20,bonding wires 21, 21A, and bond fingers 18, 18A. As describedpreviously, the latitudinal movement of molding compound under the heatsink means that a large heat sink can be placed over the top of thepackage for greater heat transfer. In fact, the heat sink could coverthe entire top surface of the substrate whereby passage means 14 wouldextend all the way to the side of the substrate. Likewise, passage meanscan many different sizes and shapes as required by the substrate andmolding compound. The passage can run from any direction in thesubstrate and need not extend all the way to the edge or to the windowas will be discussed in more detail below. There can also be more thanone passage means in accordance with another embodiment of the presentinvention.

The foregoing has described the principles, preferred embodiments andmodes of operation of the present invention. However, the inventionshould not be construed as being limited to the particular embodimentsdiscussed. Thus, the above-described embodiments should be regarded asillustrative rather than restrictive, and it should be appreciated thatvariations may be made other than those discussed by workers of ordinaryskill in the art without departing from the scope of the presentinvention as defined by the following claims.

The invention claimed is:
 1. A method for making a molded electronicpackage comprising the steps of:forming a printed wiring board havingplated via holes and a window therethrough; attaching a heat sink to afirst surface of the printed wiring board to cover the window; attachingan electronic device in the window to the heat sink; connecting theelectronic device electrically to the plated via holes; andencapsulating at least a portion of the electronic package bytransferring fluid molding compound latitudinally through a passageunder the heat sink into the window to encapsulate the electronic deviceand the connections thereto.
 2. The method of claim 1 furthercomprising:forming the passage in the first surface of the printedwiring board.
 3. The method of claim 1 further comprising:forming thepassage in a surface of the heat sink facing the first surface of theprinted wiring board.
 4. The method of claim 1 further comprisingattaching electrical contacts on the second surface of the printedwiring board to the plated vias.
 5. A method for making a molded packagehaving a potting ring comprising the steps of:forming a printed wiringboard having plated via holes and a window therethrough; attaching aheat sink to a first surface of the printed wiring board to cover thewindow; and encapsulating at least a portion of the package bytransferring fluid molding compound latitudinally through a passage toform a potting ring on the second surface of the printed wiring board.6. A method for making a molded package having a potting ring comprisingthe steps of:forming a printed wiring board having plated via holes anda window therethrough; attaching a heat sink to a first surface of theprinted wiring board to cover the window; encapsulating at least portionof the package by transferring fluid molding compound latitudinallythrough a passage to form a potting ring on the second surface of theprinted wiring board and:forming the passage in the first surface of theprinted wiring board.
 7. A method for making a molded package having apotting ring comprising the steps of:forming a printed wiring boardhaving plated via holes and a window therethrough: attaching a heat sinkto a first surface of the printed wiring board to cover the window;encapsulating at least a portion of the package by transferring fluidmolding compound latitudinally through a passage to form a potting ringon the second surface of the printed wiring board; andforming thepassage in a surface of the heat sink facing the first surface of theprinted wiring board.
 8. A method for making a molded package having apotting ring comprising the steps of:forming a printed wiring boardhaving plated via holes and a window therethrough; attaching a heat sinkto a first surface of the printed wiring board to cover the window;encapsulating at least a portion of the package by transferring fluidmolding compound latitudinally through a passage to form a potting ringon the second surface of the printed wiring board; andforming thepassage in the first surface of the printed wiring board and in asurface of the heat sink facing the first surface of the printed wiringboard.
 9. The method of claim 5 further comprising:attaching anelectronic device in the window to the heat sink; connecting theelectronic device electrically to the plated via holes; and attaching alid around the edges of the potting ring to encapsulate the electronicdevice and the connections thereto.
 10. The method of claim 5 furthercomprising:attaching an electronic device in the window to the heatsink; connecting the electronic device electrically to the plated viaholes; and encapsulating the electronic device and the connectionsthereto with a liquid encapsulant applied within an area bounded by thepotting ring.
 11. A method for making a molded electronic packagecomprising the steps of:forming a printed wiring board having plated viaholes and a window therethrough; providing a heat sink for attaching tothe printed wiring board, the heat sink having a groove in a firstsurface extending from an edge of the heat sink toward its center;attaching the heat sink to the first surface of the printed wiring boardto cover the window and such that the groove extends into the window;attaching an electronic device in the window to the heat sink;connecting the electronic device electrically to the plated via holes;and encapsulating the electronic package by transferring fluid moldingcompound over at least a portion of the first surface of the printedwiring board and latitudinally through the groove in the heat sink intothe window to encapsulate the electronic device and the connectionsthereto.
 12. The method of claim 11 further comprising attachingelectrical contacts on the second surface of the printed wiring board tothe plated vias.